The goal of this project is to evaluate the potential for improving the performance of adeno-associated virus (AAV)-based gene therapy for ornithine transcarbamylase deficiency. Ornithine transcarbamylase deficiency (OTCD) is an inherited disorder of the mammalian urea cycle, with nearly half of affected children presenting with extremely elevated blood ammonia levels within the first week of life, and a complete lack of functioning ornithine transcarbamylase (OTC) protein. This elevation of ammonia causes severe intellectual and developmental deficits when not fatal. We have developed an optimal AAV vector called the clinical candidate that provides rapid (<1day) and robust (>5 times native at the highest doses tested) and prolonged (>7 months) expression of OTC. N-carbamylglutamate (NCG) is effective at increasing urea production in patients with other urea cycle and metabolic disorders. NCG simulates the production of carbamylphosphate (CP) which is synthesized from ammonia and bicarbonate. In this way, NCG helps reduce ammonia by increasing CP formation. CP is a substrate for OTC, so increasing OTC activity will help convert CP via the urea cycle to non-toxic urea that can be safely excreted. The specific aims of this project are to 1)Show NCG will benefit a model of mild OTCD, 2) that NCG and AAV gene therapy can work together to keep ammonia levels low, and 3) long term treatment with NCG for addressing OTCD is safe. We will compare the effects of treatment with or without NCG in mouse models of mild OTCD and complete OTCD to show that NCG in combination with AAV gene therapy has therapeutic benefit by measuring increased resistance to circulating levels of ammonia, normalization of blood and tissue metabolites, and absence of markers of liver injury. We expect that gene therapy will result in patients that have their outcome improved from having complete OTCD to a milder partial OTCD. NCG treatment is expected to provide increased cognitive protection by further aiding in the removal of ammonia. Having both an effective AAV candidate and an effective pharmacological reagent puts us in a unique position to be able to test this type of innovative combination therapy. This project uses a metabolic engineering approach to study a complex system affecting multiple organs, and methods are readily translatable to other systems biology questions, and the goal is to establish a model of possible clinical treatment. The significance is that keeping blood ammonia levels low in patients with OTCD is an essential part of protecting against developmental deficits.